1. Field of the Invention
The present invention relates to a semiconductor wafer, semiconductor device and manufacturing method of a semiconductor device,
2. Related Art
Elements used in digital circuits are required to operate at high speeds in response to a high-frequency signal. On the other hand, to enhance the performance of analog circuits used in cellular phones, reduction of substrate noise (high-frequency noise), reduction of 1/f noise and high gains of high-frequency signals are required. Further, high breakdown-voltage MOS transistors used to control power sources are required to be highly resistant to voltage. There is a development of a technology for manufacturing a semiconductor device combining such digital circuits and analog circuits or high breakdown-voltage MOS transistors on a SOI substrate.
From the standpoint of substrate noise and gains of high-frequency signals, it has been found that the SOI layer for forming elements thereon had better be thin. In digital circuits, a BOX layer had better be thin as well to control carriers in the body region by back biasing.
From the standpoint of 1/f noise, however, the semiconductor layer between the bottom end of STI and the top surface of the SOI layer had better be thick. This is because channel regions of analog circuits are not connected to common ground contacts located outside STI, or resistance values between analog circuits and ground contacts increase in the case where STI penetrates the SOI layer or the semiconductor layer between the bottom end of STI and the top surface of the SOI layer is excessively thin.
Furthermore, if the BOX layer is thin, capacitance between the SOI layer and the semiconductor bulk increases, and substrate noise of digital circuits is liable to transmit to the semiconductor bulk via the SOI layer. This invites the problem that the substrate noise propagates to analog circuits via the semiconductor bulk. In addition, since a thin BOX layer invites a decrease of the breakdown voltage, a BOX layer suitable for a digital circuit is too thin for a high breakdown-voltage MOS transistor.
As such, digital circuits and analog circuits are different in adequate thickness of BOX layer from each other.
Here is provided a semiconductor substrate having a SOI layer and a BOX layer adequately controlled in thicknesses for digital circuits and analog circuits or high breakdown-voltage MOS transistor, respectively, as well as a semiconductor device and a manufacturing method thereof.